I bought a new air pressure gauge at a flea market just because it looks better than the one I'm using and is lighted, while mine is not. This new one though has two connection ports and two face needles-- a red and a white. I figured I could just cap one port if it leaked back, but it would probably not make sense that it would. Anyway, is there anything I might want to use the dual reading capability for. My current connection reads the accessory tank, which leaks down even though the main tanks remain high. I have a separate gauge in the engine compartment that reads the main system. I thought of eventually hooking this gauge up to do both. Does that sound like just busywork?

I didn't plumb mine, it was already installed from the original configuration. I can crawl "Down Under" and take a look see if you want. If I remember (and that's a big if) it's plumbed from one of the ports on the treadle valve.

You can see where the lines T off from. They ran hardline up inside the coach then went to hose up to the gauge. I'm going to use nylon with the appropriate fittings for my VDO gauge in place of the hose. I don't know how your gauge is on the back, but mine has straight male thread. I bought two kits http://www.egauges.com/vdo_indA.asp?PN=150-851 Should be here on Monday so I can plumb it up.

That gauge is for the newer two tank air systems. One is for the primary (rear) and one is for the secondary (front) air tanks.The gauges I have seen have a green needle which is for the rear tank and a orange needle which is for the front tank. They should always line up on top of each other unless you have an air leak. Of course, not having a two tank system you would cap off the secodary port to avoid a possible leak.

Cade, Could very well be, but my Eagle had a large gauge that had dual needles. It was originally set up with DD-3's, now have spring brake chambers. I just got rid of the orginal gauge and will plumb the smaller one. Even though I have spring brakes, I still like to know the pressure when I make a brake application. It's just my way for our coach.

Cade,As mentioned, this gauge has a white needle on top, and a red needle behind it. I like the idea of tying the red one to the brake. Of course, it could still be used to show the difference in the brake tank and the accessory tank.

Paul

Thanks for the picture. When I get to it, I'll have to follow the lines to be sure where they go anyway.

Lin,I will be watching this tread, as I have the same dual white/red indicator guage. I also can't get my simple mind around what information is yielded from a "application" guage for the brakes ?Will this indicate a slight leak ? I am quite conservative in spacing and brake early, before I need to floor the pedal. What am I missing, again ?

Thanks,

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Experience is something you get Just after you needed it....Ocean City, NJ

The plumbing on the RTS buses use a dual gage with white and red needles and of course two fittings on the back. They monitor the Primary and Secondary tank pressures and stay together if all is well. We RTS folks have 4-tanks. One is the wet tank fed from a dryer(AD-2), one is Rear, one is Front, one is Suspension(accessories-etc). I think an 'application pres. gauge' would be interesting because it may show some trends towards how the brakes are working together. Just a thought. Probably a 'duh' to the pros here.phil

My understanding is that older buses had the air pressure gauge reading from the accessory tank, which is after the pressure protection valve. When air brake systems changed in around 1979, the air pressure gauge was reading the dry (main) tank pressure as that was what fed the brakes directly. With modern dual tank spring brake systems there are two dry or main tanks, one each for the front and rear brakes, and there is a dual gauge to show the pressures in each dry tank.

The only pressure that I NEED to know is the pressure in the dry tank, since that is what feeds the main service brakes. If I have one gauge, that's the pressure I want to read. With a DD3 system with an emergency brake tank, if I had a secondary gauge I would want it to read the pressure in the emergency brake tank. The last pressure I would want to know about is the accessory tank pressure, sure it's interesting but kind of irrelevant to the safety systems on the bus. Reading the brake application pressure is also interesting but not critical for me, it will tell you if you have a leak or if the system is requiring greater pressure than normal to operate, which could alert you to a problem. But you can tell if you have a leak anyway by simply doing a full brake application and holding it for a minute and observing the gauge that reads the dry tank pressure. But I guess it's kind of fun to see how much pressure you are using for a stop.

So I would connect one of your gauge pointers to the dry tank, and depend on it, and if you have an emergency brake tank I would connect the other pointer to it. Since there are check valves that lock the air into the emergency brake tank, I would expect that it should read maximum system pressure at all times except when you apply the parking brake. The dry tank pressure should rise and fall with the usage of air.

FWIW, my Neoplan came from the factory with two of these dual-needle gauges. There is a separate gauge for each brake system, and on each gauge, the red needle shows system (tank) pressure, and the white needle behind it shows application pressure. When all is well, both red needles read about the same, and both white needles read about the same. This type of setup lets you know instantly if you are developing a problem in either brake system -- a difference between the gauges either in the tank or application pressures indicates a leak of some kind. If the difference is in the tank readings, the leak is in the supply system (upstream of the treadle), and if it is in the application readings it is in the service plumbing to the brake cylinders (downstream of the treadle).

Having application gauges like this also lets you become more aware of just how much of your braking potential you are using. Also, you get a good feel for how much air you actually use each time you apply the brakes.

FMVSS 121 came into effect in 1975, requiring, amongst other things, the redundant functionality that is taught in basic air brake courses: isolated dual air system, so that there is some form of braking available under a partial failure. And... the dreaded Anti-skid control.

Then it gets really muddy.

The regulation came into force over time. Some manufacturers received a permit in order to have relief from the full force of the regulations for a limited time, in order to prepare properly. The build plate inside the coach may make reference to a exemption permit.

These were VERY confused times as the government was attempting to regulate when technology was simply not sufficiently developed, and as always, too many decision makers didn't understand the complexity, and mistrusted the politics of the various advocacy groups.

And then the debacle over anti-skid control, where it was regulated, installed, failed, unregulated, disconnected, removed from new vehicles. MCI ordered every fleet owner that had bought one of the anti-skid coaches to disable the units immediately. They simply were unsafe. Coaches wouldn't stop, or they would stop by themselves.

If you have unit 31204 to 31512, it would be interesting to see if the anti-skid control "computers" are still installed in your coach. Let me know, I'd love to have a set for historical posterity.

For the MC8, MCI has air diagrams for:January 1975, prior to and under exemption from the regulation;then April 1976, a compliant anti-skid control version with air gauge plumbed to the dry tank;then December 1976, effectively back to the old way, post debacle;then April 1978, the air gauge back to the dry tank and the shuttle valve appeared.

In total, IIRC, the MC8 had at least 8 different air schematics, as there were the individual state safety requirements for coaches destined for service in New Jersey and Massachusetts to further complicate matters.

Owners of MCI products (and no doubt the rest!) through the 1970's need to be sure of which variation they have, and how much of the original plumbing is stock, how much has been bypassed, and how it has been accomplished. The guy next door's coach can't be trusted to be the same.

My MC5 pressure gauge is only connected to the accessory tank, which leaks down faster than the main system. The schematics show it that way too. I guess that the main tank reading would be the most important, in terms of safety, to add on. The application pressure seems to be of interest, but of less importance.

Sean is correct, as usual.The reason to have the application pressure is to watch for brake fade while descending a grade.If you are "applying" more and more pressure to do the same amount of slowing, it is a great indicator of how hot your brakes are getting.My model 5 Eagle came equipped with that very gauge.

I may be way off here but I always thought that was it's intended purpose.

My Neoplan has dual gauges. One for each system but both needles read that system. No applied pressure reading. Apparently just used to confirm gauge function if the needles don't match its time to check the gauge.

(snip) The last pressure I would want to know about is the accessory tank pressure, sure it's interesting but kind of irrelevant to the safety systems on the bus. (snip) Brian

Brian, auxiliary air is important to me since my transmission is air-controlled. There is a pressure regulator at 95 PSI off of that tank and the transmission should not be shifted into "go" until the auxiliary tank is up to 95 PSI. So, for my bus (which is very different in a lot of ways to what most people have), the pressure in the aux tank/system is an important thing to know/keep track of. So I do have a gauge reading that tank. My bus stock also ran the doors off the aux system but I've taken the "accordian" doors off and gone to a hinged door so that's not an issue any more.